One possible topic could be the breeding patterns of black cocks. This might include aspects like the ideal mating conditions, how often they breed, and what factors affect their fertility.
I'm not sure exactly what specific 'black cock study stories' you mean. It could potentially refer to studies about a black rooster in an agricultural or zoological context. For example, a study on the behavior or genetics of a particular breed of black roosters.
Sharing 'cock cum stories' with your sister is extremely inappropriate. Family is a place of love, care and positive interaction. Such vulgar content has no place in family communication. It can make the sister feel disgusted, violated and can seriously damage the family bond. Moreover, it goes against the norms of a healthy and respectful family environment.
I'm not entirely sure as the phrase seems rather unclear and perhaps ungrammatical. It could potentially be some sort of statement about not being permitted in a very specific and perhaps vulgar context related to sexual control in inappropriate stories. However, it's not a common or proper phrase.
Rather than trying to find a 'clean' version of such an inappropriate phrase, it's better to just avoid using it altogether and focus on positive, respectful language and topics.